1. Field of the Invention
The present invention relates to an acceleration sensor including a piezoelectric vibrator on which stress caused by acceleration is exerted.
2. Description of the Related Art
Known acceleration sensors including piezoelectric vibrators are disclosed in the following publications:
(1) Japanese Patent No. 2780594 (Vibrator Acceleration Meter);
(2) Japanese Unexamined Patent Application Publication No. 2000-266776 (Amplifying Circuit For Piezoelectric Acceleration Sensors);
(3) Japanese Unexamined Patent Application Publication No. 2000-206139 (Acceleration Detector Provided With Piezoelectric Acceleration Sensors);
(4) Japanese Patent No. 3072354 (Vibrator Acceleration Meter);
(5) Japanese Unexamined Patent Application Publication No. 8-105912 (Acceleration Sensor); and
(6) Japanese Unexamined Patent Application Publication No. 8-297138 (Piezoelectric Acceleration Sensor).
These publications are hereinafter referred to as xe2x80x9cPublication 1xe2x80x9d to xe2x80x9cPublication 6xe2x80x9d.
In general, an acceleration sensor including a piezoelectric vibrator is relatively small. Thus, the acceleration sensor has superior applicability for various uses and has a high utility value.
The vibrator acceleration meter in Publication 1 has been used as a small-sized, surface-mountable acceleration sensor. It is used as a shock sensor for hard disk drives, and as an acceleration sensor for air bags. As described above, the acceleration sensor is used for various electronic devices. As its utility value has been realized, it has been used in new ways, such as a seismometer, an inclinometer, and a rotation-detecting acceleration sensor. Accordingly, an inexpensive acceleration sensor is needed which detects acceleration of gravity and acceleration having a low frequency of several hertz or less.
The devices described in Publications 2 and 3 are low-frequency-acceleration detecting circuits used as a sensor disclosed in Publication 1, and require high-resistance resistors and special operational amplifiers. This causes a problem in that it is difficult to make the devices disclosed in Publication 2 or Publication 3 as an integrated circuit (IC), and the cost increases. Also, the devices in Publications 2 and 3 have a defect in that acceleration of gravity cannot be detected because piezoelectric effect is used to detect an acceleration signal.
The devices described in Publications 4 and 5 detect acceleration of gravity since they detect an acceleration caused change in the resonant characteristics of a piezoelectric vibrator. However, the device described in Publication 4 is more expensive because it includes a crystal vibrator, and the device disclosed in Publication 5 has an increased size because it includes a bimorph piezoelectric vibrator.
Accordingly, to enable detection of low-frequency acceleration and acceleration of gravity, it is possible to make an acceleration sensor having a resonant circuit and two piezoelectric vibrators on which reversely directional stresses are exerted by acceleration. This acceleration sensor must include piezoelectric vibrators. When a circuit disclosed in Publication 5 or Publication 6 is simply applied to the piezoelectric vibrators, the following problems occur:
(i) The ratio between the impedance of an external resistor and the impedance of a piezoelectric vibrator is used as a signal. Thus, when the resonant frequency is high, the capacitance (component which is not changed by acceleration) of the piezoelectric vibrator operates to lower the impedance. Accordingly, this reduces the sensitivity of the impedance component that is not changed by acceleration.
(ii) Although a method of amplifying an amplitude signal is used, a special circuit (active elements) is required to accurately amplify a voltage having a high frequency of several megahertz to several tens of megahertz, such that the required cost increases. In addition, it is difficult to make the circuit as an IC.
(iii) To amplify a voltage having a high frequency of several megahertz to several tens of megahertz, the parasitic capacitance of a circuit substrate must be controlled. Accordingly, after the circuit and its elements are integrated (module structure), the sensitivity must be adjusted, and the adjustment of only the elements cannot guarantee the desired characteristics.
(iv) The parasitic capacitance of the circuit substrate changes due to contamination on the substrate, humidity, and deterioration of the substrate. Accordingly, to ensure reliability, coating or hermetic sealing packaging is required.
(v) An acceleration signal is output in the form of an amplitude modulated (AM) waveform. Thus, it is difficult to increase the precision of AM demodulation at high frequencies, such that the required circuit is complex.
To overcome the above-described problems, preferred embodiments of the present invention provide a small-sized, inexpensive acceleration sensor which detects low frequency acceleration and acceleration of gravity without experiencing problems described above.
According to a preferred embodiment of the present invention, an acceleration sensor preferably includes first and second piezoelectric vibrators in which reverse stresses are generated by acceleration, a bridge circuit which includes a first impedance circuit including the first piezoelectric vibrator, a second impedance circuit including the second piezoelectric vibrator, a first load impedance circuit including a first capacitor, and a second load impedance circuit including a second capacitor, and in which the first impedance circuit and the second impedance circuit are connected to a first junction point, the first impedance circuit and the first load impedance circuit are connected to a second junction point, the second impedance circuit and the second load impedance circuit are connected to a third junction point, and the first load impedance circuit and the second load impedance circuit are connected to a fourth junction point having a potential used as a reference potential, a voltage-dividing impedance circuit provided between the second junction point and the third junction point, a feedback-signal processing circuit for feeding back the signal of a voltage-dividing point of the voltage-dividing impedance circuit to the first junction point, wherein an oscillating circuit is arranged, and a phase-difference-signal processing circuit which detects an oscillation-output-phase difference between the second junction point and the third junction point and which outputs an acceleration-detection signal.
In the acceleration sensor, the piezoelectric-vibrator-included impedance circuit and the capacitor-included impedance circuit may be substituted for one another.
Preferably, the feedback-signal processing circuit includes an amplifier for amplifying the signal of the voltage-dividing point and a phase-shift circuit for shifting the phase of the feedback signal.
The voltage-dividing impedance circuit is preferably a voltage-dividing circuit defined by connecting a plurality of resistors in series.
The phase-difference-signal processing circuit includes a first phase-difference detecting circuit which detects a phase difference between a reference signal and the signal of the second junction point while using the signal of the voltage-dividing point as the reference signal, a second phase-difference detecting circuit which detects a phase difference between the reference signal and the signal of the third junction point, and a differential amplification circuit for performing differential amplification on the outputs of the first and second phase-difference detecting circuits.
Alternatively, the phase-difference-signal processing circuit may include a first phase-difference detecting circuit which detects a phase difference between a reference signal and the signal of the second junction point while using the signal of the first junction point as the reference signal, a second phase-difference detecting circuit which detects a phase difference between the reference signal and the signal of the third junction point, and a differential amplification circuit for performing differential amplification on the outputs of the first and second phase-difference detecting circuits.
Alternatively, the phase-difference-signal processing circuit may include a first phase-difference detecting circuit which detects a phase difference between a reference signal and the signal of the second junction point while the signal of the sixth junction point to which the phase-shift circuit and the amplifying circuit in the feedback-signal processing circuit are connected is used as the reference signal, a second phase-difference detecting circuit which detects a phase difference between the reference signal and the signal of the third junction point, and a differential amplification circuit for performing differential amplification on the outputs of the first and second phase-difference detecting circuits.
The acceleration sensors may further include a phase adjusting circuit for adjusting the phase of the reference signal which is input to the first and second phase-difference detecting circuits.
The oscillation circuit is classified as an LC-resonance oscillation circuit which uses the inductive component in the range of between the resonant frequency and the anti-resonant frequency of a piezoelectric vibrator and a capacitor. The oscillation circuit is not substantially affected by the parasitic capacitance of a circuit substrate because capacitors having relatively large capacitance are used as loads. Accordingly, high reliability is ensured without using coating or hermetic sealing packaging. In addition, the desired characteristics of the acceleration sensor as a single device are achieved.
According to the above-described structure, no deterioration in sensitivity occurs due to a high frequency because the sensitivity increases in proportion to the Q value of the resonance. Thus, desired detection sensitivity is obtained, even if small piezoelectric vibrators are used, such that the entire acceleration sensor is substantially reduced in size.
The phase-difference detecting circuit is defined by a binary logic circuit because it detects a phase change, such that it is suitable to be used in high frequency ranges. Accordingly, it can be inexpensively produced as an IC, even when the resonant frequency is set in the range of, for example, several megahertz to tens of megahertz.
Both the bridge circuit and phase-difference-signal processing circuit in the oscillation circuit are differential types. Thus, noise components having the same phase and characteristic changes caused in circuits are cancelled.
When the acceleration sensor is constructed as an IC, additional advantages are obtained because the relative variation of each element of the IC is very small. Thus, the circuit configuration of the acceleration sensor is suited for the IC structure.
In an acceleration sensor according to preferred embodiments of the present invention, a phase adjusting circuit is provided at the input side of the first and second phase-difference detecting circuits. The phase adjusting circuit is used for optimizing the phases of the reference signals, which greatly increases the precision of detecting acceleration.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments thereof with reference to the attached drawings.